How often can one be wrong in this place?
When properly prepared for (meaning, you know you are going into the tank), sensory deprivation experiences are quite enlightening as to the inner working of your mind. Only when abused and forced on unsuspecting people can it cause ill-effects as the mind is unaware of what has happened to the normal buzz of stimuli. The movie Altered States will drive you stark-raving-mad, regardless.
Gravity is a fictitious force.